1. Field of the Invention
This invention relates to a radio frequency resonator and a method for producing the same, and more specifically, to a radio frequency resonator made of aluminum, which is used in accelerators, beam irradiation apparatuses, and ion implantation apparatuses etc. and to a method for producing the same.
2. Description of the Prior Art
Radio frequency resonators made of aluminum are used as main units in accelerators, beam irradiation apparatuses, ion implantation apparatuses, etc. An example of an ion implantation machine used for implanting ions into silicon wafers 8 is shown in FIG. 1, and has a linear accelerator 14 employing a number of the aluminum radio frequency resonators 20 as shown in FIG. 2. In the drawings, the numeral 10 denotes an ion source, 12 and 16, analyzer magnets, and 18, a scanning unit.
As shown in FIG. 3 in detail, a container 24 of the radio frequency resonator 20 mainly comprises: a block 25, a dished head 26, a wall 28, and a top 30. The block 25 and dished head 26 are fitted to the wall 28, at the side of a cylindrical electrode 22 to be inserted into a beam passage of the linear accelerator 14. The wall 28 is composed of a single-layered cylinder. The top 30 is welded to the wall 28 at the opposite side to the electrode 22. The container 24 as constituted has three weld seam lines W.
This container 24 has been fabricated so far, as illustrated in FIG. 4 in detail, in a process comprising: forming a wall 28 by bending a rectangular flat plate 32 by means of rolls 34, followed by seam welding with a welding torch 36; spinning a disk-shaped plate 42 into a dished head 26 by means of a spatula tool 46 onto a die 44 to follow its shape; welding the dished head 26 to the wall 28 with the welding torch 36; and welding a top and block to the wall 28.
Herein, all of the dished head 26, wall 28 and top 30 have been made of aluminum alloys such as A5052 and A6061-T6, with rather high electrical resistance, for keeping mechanical strength.
In addition, in order to reduce the power loss caused by this electrical resistance, the inner surface of the container has been plated with a metal of low electrical resistance such as copper and silver by plating with masking of the outer surface of the container. The reasons why only the inner, but not outer, surface has metallic coating is that the outer surface, if copper or silver plated, would easily be oxidized or discolored when touched with hand, which eventually would result in the deterioration in appearance.
However, the radio frequency current, flowing on the resonator inner surface, is limited within a depth, the skin depth, which depends on the frequency from the skin effect. Therefore, metal-coated layer is usually about twice as thick as the skin depth, which becomes thicker corresponding to lower frequency applications. A low radio frequency application in 13.56 MHz, for example, needs a plating thickness of around 40 μm. However, it is practically impossible to keep the plating thickness with enough uniformity and reproducibility, over the entire resonator inner surface with complicated shape. Furthermore, plating thickness is very difficult to be inspected in quality control. Consequently, the plating thickness is liable to differ by each individual container, which eventually causes individual differences in the radio frequency loss among resonator containers.
Theoretically, the electrical conductivity is clearly higher in the plated layer, if thick enough, than in the substrate. However, if the plated layer is thinner than the skin depth, the electrical conduction is shared with structural material of the container, an aluminum alloy, and eventually, the apparent electrical resistance increases. Due to the difficulty in thickness control, the performance of resonators eventually depends on the uncontrollable plating thickness, which has been a fundamental drawback so far.
In addition, the hardness of aluminum alloy like A6061-T6 used in a resonator container cannot allow the dished head and wall to be formed integrally in one body, and the low thermal conductivity of the alloy causes surface temperature rise, to invite, eventually, unfavorable increase in electrical resistance.
Furthermore, there have been problems of high production cost because the production of resonator containers comprises welding of many components and metal plating with masking, both of which need a lot of manual labor, resulting in much processing time (man-hour).